The invention relates to a current source circuit arrangement comprising a first current path extending between a first terminal and a common terminal and including a current source and the collector-emitter path of a first transistor, and a second current path extending between a second terminal and the common terminal and including the collector-emitter path of a second transistor, which has a base electrode commoned with the base electrode of the first transistor and is of the same conductivity type as the first transistor.
Such current source circuit arrangements, which are also called current mirror circuits, are frequently used in electronic circuit arrangements. These current source circuit arrangements can be used especially in integrated power amplifiers for audio applications.
In the simplest form of such a current source circuit arrangement, the first transistor in the first current path is connected as a diode. When the first and the second transistor are identical, the current flowing through the second current path is substantially equal to that flowing through the first current path because, due to the commoned base electrodes, the base-emitter voltages of the two transistors are equal. The current in the second current path can also be made larger or smaller than the current in the first current path by scaling the emitter areas of the first and second transistors or by including unequal resistors in the emitter leads of the first and second transistors. By adding a transistor, the current in the second current path can be made more equal to the current in the first current path. In one version of this configuration, the base current of the first and second transistors can then be supplied by a further transistor, whose emitter is coupled to the commoned base electrodes of the first and second transistors and whose base electrode is coupled to the collector of the first transistor.
Further, additional output currents can be obtained by connecting transistors with their base-emitter paths in parallel with the base-emitter path of the second transistor.
In such current source circuit arrangements, however, the current in the second current path strongly depends upon variations in the voltage at the common terminal which is usually connected to the positive or negative supply voltage. There is present between the commoned base electrodes and ground (the substrate in the case of an integrated circuit) a parasitic capacitance which constitutes a shortcircuit for high frequencies. This is especially the case for lateral pnp transistors, in which the base is constituted by an epi region which has a comparatively large parasitic capacitance C to the substrate. In the case when the current source circuit arrangement of the kind described is provided with a further transistor, this effect is increased by the presence of the parasitic capacitance between the base electrode of this further transistor and the substrate. As seen at the emitter of this further transistor and consequently at the commoned base electrodes of the first and second transistors, this capacitance has an apparent value, which is .beta.+1 times larger than its actual value .beta. being the current amplification factor of this transistor. Variations of the voltage at the common terminal, for example in the form of an alternating voltage modulated onto the supply voltage, result due to these parasitic capacitances in variations of the base emitter voltages of the first and second transistors, which in turn lead to variations of the current in the second circuit.
Variations of the voltage at the common terminal result in variations of the output currents of the current source circuit arrangement, which may adversely affect the operation of circuitry to which it is connected. One of the applications in which this influence gives rise to problems is that of integrated power amplifiers in which so-called "bootstrapping" is utilized for obtaining a large dynamic range from the output transistors. Such an amplifier is, for example, the integrated circuit of the type TDA 1015 described in Philips Data Handbook "Integrated Circuits", Part 1, January 1983. In such an amplifier, a so-called bootstrap line is connected through a resistor to the positive voltage supply line. Current source circuit arrangements of the kind described may then be used inter alia as a load for the drive amplifier for the output stage and as a current source for the bias current adjustment of the output stage. The common terminal of the current source circuit arrangement is then connected to the bootstrap line. The larger dynamic range from the output transistors is obtained since the alternating voltage signal at the output of the amplifier is passed via a bootstrap capacitance to the bootstrap line. Due to the presence of the parasitic capacitances, however, the current from the current source circuit arrangement will then also comprise an alternating current component which is converted at the high impedance input of the output stage into a comparatively large alternating voltage, which in turn appears at the output of the output stage. The signal has then traversed a positive feedback loop. At a frequency determined by the value of the parasitic capacitances of the current source circuit arrangement the loop amplification becomes higher than unity, as a result of which instabilities and oscillations occur.
It is known to avoid these effects by connecting a capacitance in parallel with the resistor between the voltage supply line and the bootstrap line, as a result of which the bootstrap line potential is smoothed for high frequencies. However, this capacitor should have a capacitance of a few hundred nF, so that it cannot be integrated, which results in additional cost due to the required additional connection to the integrated circuit. Further, this capacitor leads to an increase in the interference radiation from the integrated circuit.
Another known method of avoiding these instabilities and oscillations is to connect the compensation capacitance, which also for reasons of stability is generally arranged between the output of the output stage and the input of the drive stage, not to the output, but to the input of the output stage. Thus, for high frequencies the input impedance of the output stage is very much decreased, so that the alternating current component of the current source circuit arrangement can find a low-impedance path to ground. However, when the input of the output stage becomes low-impedance, the disadvantage occurs that the so-called "cross-over distortion" is adversely affected for high frequencies. Further, the signal path via the current source for the bias current adjustment is still present so that instabilities can continue to occur.